A well known semiconductor component is semiconductor memory, such as a random access memory (RAM). RAM permits repeated read and write operations on memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors which require periodic refreshing to maintain the stored data.
In recent years, the number and density of memory elements in memory devices have been increasing. Accordingly, the size of each element has been shrinking, which in the case of DRAMs also shortens the memory element's data holding time. Typically, a DRAM memory device relies on element capacity for data storage and receives a refresh command in a conventional standardized cycle, about every 100 milliseconds. However, with increasing element number and density, it is becoming more and more difficult to refresh all memory elements at least once within a refresh period. In addition, refresh operations consume power.
Recently programmable conductor memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory elements. Kozicki et al. in U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796, discloses a programmable conductor memory element including an insulating dielectric material formed of a chalcogenide glass disposed between two electrodes. A conductive material, such as silver, is incorporated into the dielectric material. The resistance of such material can be changed between high resistance and low resistance states. The programmable conductor memory is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes. The mechanism by which the resistance of the memory element is changed is not fully understood. In one theory suggested by Kozicki et al., the conductively-doped dielectric material undergoes a structural change at a certain applied voltage with the growth of a conductive dendrite or filament between the electrodes, effectively interconnecting the two electrodes and setting the memory element in a low resistance state. The dendrite is thought to grow through the resistance variable material in a path of least resistance.
No matter what the mechanism, the low resistance state will remain in tact for days or weeks after the voltage potentials are removed. Such material can be returned to its high resistance state by applying a reverse voltage potential between the electrodes of at least the same magnitude as used to write the memory element to the low resistance state. Again, the highly resistive state is maintained once the voltage potential is removed. This way, such a device can function, for example, as a resistance variable memory element having two resistance states, which can define two logic states.
Since the typical non-programmed state of a resistance variable memory element is the high resistance state, the memory element is programmed by an applied voltage to place the memory element into a low resistance state. To provide added memory design flexibility, it would be desirable to have a resistance variable memory element that is in a low resistance state when not programmed and which switches to a high resistance state when programed.